Nitrogen is a plentiful element in the biosphere, accounting for 78 percent of the air around us. It is essential for life, being a building block for proteins that all organisms need.
The Romans recognized the need for nitrogen indirectly by noticing that grain crops produced greater yield when planted after certain crops, like lupin, that we now know to be legumes. German chemist Justus von Liebig identified nitrogen as a key mineral element for plant growth in the late 1800s. And the Green Revolution of the 1960s relied heavily on nitrogen fertilization of soils and crop varieties that could take advantage of this. Yet, nitrogen’s role in the food system continues to challenge us (as growers, consumers, and environmentalists) despite decades of research and development.
The most obvious challenge is the need to supplement our crops with nitrogen fertilizer, be it from organic or inorganic sources. Crop plants (with the exception of legumes) cannot directly utilize the abundance of nitrogen in the air around them. Instead, they must get their nitrogen from the soil through root uptake (or in limited quantities from foliar applications).
In natural ecosystems, the soil nitrogen reserves and turnover, and the plant biomass being supported, are in balance. Soil organic matter is the storehouse, and soil organisms are the engine, to convert nitrogen from organic sources to the inorganic (mineral) forms on which plant roots rely. Roots can take up organic forms of nitrogen, but for this process to be meaningful, soil nitrogen levels must be far below what would be practical for yields needed by farmers. Native plant communities are generally polycultures containing a legume that can convert nitrogen in the air into a plant-available form in its roots through a symbiotic association with Rhizobium bacteria. Some nitrogen is also deposited annually from the atmosphere (from lightning, volcanic emissions, and fossil fuel combustion), and free-living, nitrogen-fixing microbes in the soil can contribute a small amount.
But things are different in an agricultural setting. We often ask the soil to produce higher yields, and we export nutrients in the food harvested and shipped from the farm, with no return of nutrients from the end users (people). By breaking the nutrient cycle, we take a solution and create two problems, where we have to bring in nutrients (fertilizers) each year for the crops, and we have to dispose of nutrients (municipal wastewater) that accumulate in cities.
We continue to be challenged by nitrogen efficiency. Most agricultural systems appear to be inherently "leaky" for nitrogen. A 50 percent efficiency (percent of nitrogen applied as fertilizer that actually ends up in the crop) in an agricultural crop is considered high, with many crops in the 20 to 30 percent range. The remaining nitrogen in the soil is subject to loss. This can be by water (runoff or leaching) and contribute to the hypoxia, or "dead zones," in places like the Gulf of Mexico. Nitrogen can also be lost in gaseous form from the soil, some of which is nitrous oxide (N2O), a greenhouse gas 200 times as potent for trapping heat as carbon dioxide. Technologies that can increase nitrogen use efficiency and reduce losses to the environment are needed.
Right now, world food production is heavily reliant on synthetic nitrogen fertilizer.
The Haber-Bosch process for manufacturing it was developed in the early 1900s. Today, it is used to produce 500 million tons of nitrogen fertilizer a year, consuming 1 percent of the world’s energy supply, and supporting 40 percent of the world population. Haber and Bosch both won Nobel Prizes and are considered by the journal Nature to be the most influential people of the twentieth century (not Albert Einstein or Bill Gates!), as world population could not have grown as it did without their discovery (see "Population growth and synthetic N fertilizer"). Their process uses natural gas (methane) as a feedstock to combine with atmospheric nitrogen to form ammonia. Natural gas is essentially a nonrenewable resource with many competing uses.
Nitrogen fertilizer prices doubled over the past several years, in part due to rising demand, natural gas prices, and transportation costs, according to a report by U.S. Department of Agriculture economist Wen-yuan Huang. The United States now imports 50 percent of its nitrogen fertilizer from Canada, Trinidad, Russia, and the Middle East. So, the future of nitrogen fertilizer has uncomfortable parallels with petroleum. Future supply and price are likely to experience increasing volatility. Thus, now is a good time to be seriously developing a long-term strategy to supply nitrogen to agriculture.